import init;
1. Electrostatics¶
1.1 Goals¶
To understand and verify the behavior of the two kinds of charge, denoted “positive” and “negative”, respectively.
To understand the response of the electroscope when a charged rod is brought near, so that the electrical charges on the rod interact with charges already present in the electroscope.
To visualize charge transfer between charged rods, the electroscope, and other objects, and to understand how the electroscope is used to compare the net charges on two objects.
1.2 Introduction¶
Electroscopes are used to detect the presence or absence of electric charge. They come in various forms, but a picture of a typical electroscope is shown in Figure1. Inside the electroscope a metal needle can pivot on a wire support shaped something like a paper clip. This structure inside the electroscope is connected to the outside by a metal rod passing through a plastic insulator. The metal disk on top simply allows charge to be detected more efficiently; otherwise its geometry is not too important. The term “electrostatics” refers to charges that are basically stationary, rather than continuously moving as in a wire carrying an electric current. An analogy may be made to water in a bathtub as opposed to a flowing stream of water.
Some important things to remember are:
Electric charges come in two varieties that are designated positive and negative.
Charges of the same variety repel one another while charges of the opposite variety attract one another.
Charges exert greater forces on one another when closer together (Coulomb's law).
All materials are composed of positive and negative charges.
In metal objects, a small fraction of the negative charge is relatively free to move from one place to another within the object. (This is why metals are called conductors.)
Electric charges in insulators such as rubber and glass are essentially fixed in place.
The positive charges in solid materials are in the atomic nuclei and are not free to move.
Electric charges in static equilibrium have no net force acting on them.
When rubbed with silk, a glass rod acquires a net positive charge on its surface by giving up electrons to the silk, which has a stronger affinity for electrons.
The plastic (polyvinyl-chloride, or PVC) acquires a net negative surface charge when rubbed with wool by “stealing” electrons from the wool.
**Caution: The glass rod is brittle. Return it to the tray when not in use. If placed on the table, the rod can roll off and break. Avoid handling the glass rod, the plastic tube, and the wool and silk fabrics any more than necessary. Their electrostatic properties are degraded by moisture and oil from your hands.**
1.3 Holding a charged rod close to the electroscope plate¶
Ground the electroscope as illustrated in Figure 1. This works because your body can absorb or give up small amounts of charge without suffering any ill effects. You could use a wire connected to the earth (or ground, hence the term “ground”), but your body is handier in this case. Rub the glass rod to charge it, and then move it close to the circular disk on top of the electroscope without touching the disk with the rod. What do you observe? As the rod is moved away from the disk, what happens? Hypothesize what is happening to the charges. If at any time you suspect that the needle is stuck, gently tap the case of the electroscope with your finger. The case is not connected to the top plate. Tapping the case will not affect the charge on the plate or needle.
Repeat the same sequence with the plastic tube after rubbing it with wool. Again record your observations. (Note: rubbing the plastic pipe with wool can produce enough electrostatic charge to produce tiny sparks when the pipe is brought near the plate of the electroscope. Sparks transfer charge to the electroscope. The effect of charge transfer is the subject of the next experiment. If you suspect a spark, ground the electroscope and repeat the experiment without bringing the rod so close. The needle can deflect considerably before a spark occurs.)
Now explain your hypothesis with the aid of some simple “cartoons”—a series of pictures with words of explanation; your TA will have some helpful suggestions for making simple drawings. Show what the electric charges on the electroscope are doing as the charged rods are brought close and then moved away. You will need a sequence of several cartoon pictures to show the locations of the charges on the electroscope for different positions of each rod. If you can't support your hypothesis by your observations and pictures, you may need to make another hypothesis.
1.4 Charging the electroscope by direct contact¶
Ground the electroscope again. This time touch the charged glass rod to the disk, and then move the rod away. What happens to the needle of the electroscope? Make a hypothesis about what happened when you touched the disk with the rod using some “cartoons” as visual aids. Without grounding the electroscope, test your hypothesis by bringing the charged glass rod near the disk at the top of the electroscope but without touching it. What happens to the electroscope needle? Explain whether this observation supports your hypothesis or not. If the observation doesn't support your hypothesis, redo the whole procedure and make sure that the observed behavior is repeatable—an important aspect of the scientific process. Record all your hypotheses, whether they turn out to be correct or incorrect. By using the scientific method we hope to reach the correct explanation in the end. If the behavior is repeatable, then make another hypothesis to explain your observations and test it again. To double check your understanding, bring the plastic tube close to (but not touching) the electroscope that was touched at the outset with the charged glass rod and observe what happens. Is your hypothesis consistent with these additional observations? Explain with the aid of another cartoon sequence.
Repeat the entire process outlined in the previous paragraph, but start this time by touching the initially uncharged electroscope with the charged plastic tube.
Can a net electric charge be left on the electroscope by touching it with the rods? How does the sign of the charge on the electroscope compare to the charge on the rod that touches it? Summarize your findings for this exercise.
1.5 Charging the electroscope by induction¶
Ground the electroscope again, as in Figure 1. With your finger still touching the edge of the disk and your thumb still touching the body of the electroscope, bring the charged glass rod up close to the the other side of the disk (away from your finger) without touching the disk. Now remove your finger from the disk first and then move the glass rod away. What do you observe on the electroscope? Make sure that it is repeatable. Make a hypothesis about what happened to the charge in the electroscope and record it. Then test your hypothesis using what you learned so far. Modify your hypothesis as necessary. Explain your reasoning with another cartoon sequence.
Now beginning with the charged plastic tube repeat the process described in the previous paragraph. Summarize your results for this section.
1.6 Effect of lit match on electroscope charge¶
Using your knowledge of the behavior of the electroscope and the charged rods, determine what variety of charge is released when a match is burning. Hold the burning match about 2 cm above the disk. (Hold the match at least 1 cm from the plate.) Try it with the electroscope initially uncharged, positively charged, and negatively charged. Explain in detail your procedure, results, reasoning, and conclusions. More cartoons are needed here.
1.7 Summary¶
Summarize your findings concisely. Provide a brief explanation of your most important observation in each experiment.
Holding a Charged Rod¶
Fig. 1: When the positively charged glass rode was brought close to the metal disk the needle turned to 45 degrees from the to the metal support and returned to normal when the glass rod was removed The negative charges are being attracted to the metal disk by the glass rod which leaves the positive charges on the needle causing the needle to become almost perpendicular to the metal support. Fig. 2: When the negatively charged PVC rob was brought close to the metal disk the needle became almost 45 degrees from the metal support. The positive charges gathered on metal disk near the PVC rod that was charged negatively and the negative charges were left on the needle which resulted in the needle turning to a 45 degree.
Charging the Electroscope¶
Fig. 3: When the positive glass rod touches the metal disk the needle remains at about a 45 degree angle from the metal support indefinitely. The glass rod transfers some of the negative charge to the metal disk which causes the needle to remain at an angle for an extrended period of time. Fig. 4: When the negative rod was brought closer a second time the needle did not move more than a millimeter which suggests that the metal disk was almost as negatively charged as the glass rod. After the negatively charged PVC was brought into contact with the metal disk the needle did not remain at the same degree as when the rod was not touch. However, the needle did not return to the original starting position. The results support the hypothesis that the needle will change position based on the relative charge of the disk to the rod that was used in the interaction. The charge of the metal disk is similar to the charge of the rod after they have interacted.
Charging Through Induction¶
Fig. 5: After the fingers and glass rod have been removed the needle moved about 20 degrees from the resting point and remained at that point. When the process was repeated the results were very similar with only a small varying degree of movement. Fig. 6: After the fingers and PVC were taken away from the metal disk the needle turned to about 45 degrees from the resting point and remained there. When the process was repeated with the PVC rod the results were almost the exact same. The results from this section were supportive of the hypothesis that the metal disk was made neutral when fingers were grounding the disk and the rod was present but when the fingers and rod were removed the needle was left with a charge that depended on the rod to make the needle become neutral.
Effect of a Lit Match¶
Fig. 7: The uncharged metal disk did not show any change when the lit match was hold above the plate at about 2cm and then 1cm. Fig. 8 & 9: The positively and negatively charged electroscopes became neutral when the lit match was held about 2cm about the metal disk. The charged trials began with charging a glass or PVC rod with the appropriate cloth and touching the rod to the metal disk. After the charge was imparted on the metal disk the lit match was brought about 2cm from the top of the plate and resulted in needle returning to the neutral position for each trial. The flame causes the elements around it to become ionized and these ions absorb the charge of the electroscope regardless of the electroscope's charge.
